Why aren’t domed heads included in simple pressure vessels?
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Ladies and gentlemen, TSG21-2016, Annex A, P102 – Definition of simple pressure vessels: (1) A pressure vessel is composed of a cylinder together with a flat cover and a convex head (excluding domed heads), or it consists of two convex heads； (2) The materials of the main pressure-bearing components such as the cylinder body, head, and nozzles are carbon steel, austenitic stainless steel, or Q345R; (3) The design pressure is less than or equal to 1.6 MPa; (4) The volume is less than or equal to 1 cubic meter ； ………… I would like to ask why domed heads are not included in item (1); is there anything special about domed heads? The cause was not found.
This post was last edited by Shutong on 2020-7-31 at 19:09. The dome head is also known as a seamless spherical head. In this type of head, part of the spherical shape is directly connected to the cylinder; it has a simple structure and is easy to manufacture. It is commonly used as an intermediate head between two separate pressure chambers in containers, or it can also be used as an end cover. The fillet welds at the junction of the head and the cylinder should have a fully penetrated structure. At the connection between the spherical head and the cylinder, the radius of curvature changes abruptly, and since the two shells do not share a common tangent, a lateral thrust is generated, resulting in considerable discontinuous stresses. Such heads are generally only suitable for applications with low pressures.
The T-joint where the spherical head is connected to the cylinder must have a fully penetrated weld structure. On January 1, 1993, an explosion occurred in the re-refining oil tank and the wax oil tank of Unit 2 at Cangzhou Refinery... After the accident, it was found that 12.5% of the welds between the intermediate spherical head and the connecting cylinder had serious defects. One of the sections is 500 mm long with incomplete welding, and in the adjacent 350 mm long weld, a 12 mm rebar was trapped inside; the workers handled it carelessly. This accident shows that if attention is not paid to weld quality, it can lead to a major disaster. During design, an appropriate groove type for the T-joint should be selected, and the welding and non-destructive testing requirements for such welds must be defined to ensure welding quality. This weld is generally subject to 100% magnetic particle testing, and the results shall meet the requirements of JB/T4730.5 \"Non-destructive testing of pressure equipment – Part 4: Magnetic particle testing\", with a grade of I as acceptable.
At the junction between the spherical head and the cylinder, there are very high bending stresses and transverse shear forces (collectively referred to as edge stresses). This issue cannot be resolved using the moment-free theory, and the overall stress levels are quite high, making it relatively dangerous. Since simple pressure vessels have lower requirements in terms of manufacturing, they are excluded from consideration for safety reasons.
The dome-shaped head is not excessive; the stress conditions are complex
Why not? It’s specified in the technical standards; there’s no need to ask such a question. If you think for yourself, all the answers above are excellent; moreover, this type of end cap is not a common one. Simple pressure vessels are those with simple structures, low parameters, and modest requirements.